The housing of piezoelectric crystals, semiconductor components, quartz crystal oscillators, relays and electrical assemblies must be performed hermetically in many applications. When the housings are sealed shut by such common technical methods as brazing, soldering and resistance welding, trace impurities are formed within the capsule (brazing vapors, for example), which may impair the efficiency of operation of the component mounted within the housing and, thus, detract from the component's quality and reliability.
The present invention relates in general to electronic component mounting arrangements and more particularly to a low profile crystal header package with an improved crystal mounting arrangement which facilitates multiple electrical connections to each surface of the crystal plate without flexing or repositioning the crystal plate. The present invention is directed to that segment of the market where high reliability is very important.
It has long been the practice to hermetically seal the protective metal crystal header package. Once sealed, the crystal package can be used in many different environments with a high degree of confidence that the crystal plate mounted therein will reliably perform at the desired frequency.
In recent years, space limitations have required smaller and smaller crystal mounting assemblies, which has caused an increase in the electrical and mechanical problems relative to properly mounting a crystal blank within a crystal header package, while not causing damage to the crystal blank, maintaining frequency standardization, and avoiding internal environment contamination during the sealing process. Likewise, as crystal applications have become more and more complex, more mechanical and electrical contact points are required, on smaller and smaller crystal blanks. Thus, it has been the prior art practice to provide a small crystal package which allows the mounting of the crystal blank within a metal evacuated container, such container having terminal pins projecting within the internal cavity where the pins are connected via wire leads to the electrodes found on the crystal blank. These same terminal pins also serve to support the crystal blank within the evacuated container as its seams are sealed either by soldering, hot welding or cold welding technologies.
The use of high pressure cold welding technology has found favor in the prior art since cold welding has the primary advantage of being free from fumes and contamination. In cold welding, a molecular bond is obtained by a cold flow of metal under extremely high pressures without heat. Because of the pressures involved, cold welding has an occasional disadvantage of causing distortion to the metal container during the welding operation, which may result in the internal terminal pins being displaced from their intended positions. Since the crystal blank is secured by the terminal pins, distortion of the pins will cause a stress to be applied to the crystal blank, which will adversely affect the frequency characteristics of the crystal blank.
In the prior art there are various types of crystal mounting apparatus that are commonly used. In one approach, the quartz crystal blank is supported by a center mounting post, which allows horizontal mounting of the crystal and reduces overall height of the crystal package assembly. A disadvantage of this type of crystal mounting approach is that the crystal blank needs to be adequately supported during the wire bonding process so as to withstand the surface pressures involved without sustaining damage to the crystal blank. During assembly of the crystal wafer onto its respective center post mounting position, it is desirable to avoid overstressing the wafer. Such stresses can occur by the means which are used to mount the crystal wafer in the crystal package or by the mechanical stresses which develop during the hermetic sealing of the crystal package itself.
In addition to sealing the component parts used to make the crystal housing, the prior art has also been concerned with sealing the interface between the housing and the terminals. With regard to this type of sealing, the prior art generally consisted of the steps of hermetically sealing a pair of terminals in an eyelet to form a header, mounting a crystal wafer on an associated header, and hermetically sealing the header to a metal or glass container with the crystal wafer positioned within the container. Headers have been characterized either as matched glass or compression glass headers. In a glass header, a single vitreous material is used to fabricate the terminals and eyelets, while a second vitreous material which has the same thermal coefficient of expansion as the eyelet and terminals is used to seal the terminals to the eyelet. Generally, the eyelet and terminals are made of an iron-nickel-cobalt alloy material such as KOVAR and the vitreous material is a glass which has been selected to have a thermal coefficient of expansion substantially the same as that of KOVAR.
The sealing process is facilitated by placing molten glass in the eyelet around the terminals and then cooling the glass to cause it to shrink and tightly grip the terminals. The similar coefficients of thermal expansion of the KOVAR components and the vitreous material insure that the seal between the terminals and the eyelet will be maintained. In both the matched and the compression glass headers the integrity of the terminal to eyelet seal depends upon the attainment of a good seal between the terminal and the glass.
There is thus a need for a low-profile crystal header package that has an extremely low leakage rate of contaminants while exhibiting a high degree of crystal frequency stability. The present invention is directed toward filling that need.